1. Field of the Invention
The present invention relates generally to optical fibers, and more specifically to optical fibers with low bend loss.
2. Technical Background
Optical fibers formed of glass materials have been in commercial use for more than two decades. Such optical fibers are typically comprised of silica doped with one or more dopants, such as germanium, fluorine, and/or other dopants, to achieve a refractive index profile that results in guiding light signals along the length of the fiber. Other optical fiber is assisted by the presence of holes in the silica, or hole assisted optical fiber, which includes holes or voids running longitudinally along the fiber axis, such as in microstructured optical fiber. The holes generally contain air or an inert gas, but may also contain other materials. The majority of hole assisted optical fiber have a plurality of holes located around the core, wherein the holes continue for a relatively long (e.g. for many tens of meters or more) distance along the length of the fiber, and typically the holes extend along the entire length of the optical fiber. These cladding holes are also most typically arranged in a regular, periodic formation around the core of the optical fiber. In other words, if cross sections of the optical fiber are taken along the length of the optical fiber, the same individual holes can be found in essentially the same periodic hole structure relative to one another. Examples of such microstructured fibers include those described in U.S. Pat. No. 6,243,522.
Microstructured optical fibers may be designed to have a wide variety of properties, and may be used in a wide variety of applications. For example, microstructured optical fibers having a solid glass core and a plurality of holes disposed in the cladding region around the core have been constructed. The position and sizes of the holes may be designed to yield microstructured optical fibers with dispersions ranging anywhere from large negative values to large positive values. Such fibers may be useful, for example, in dispersion compensation. Solid-core microstructured optical fibers may also be designed to be single molded over a wide range of wavelengths. Most solid-core microstructured optical fibers guide light by a total internal reflection mechanism; the low index of the holes acts to lower the effective index of the cladding region in which they are disposed.
Micro-structured optical fibers are typically manufactured by the so-called “stack-and-draw” method, wherein an array of silica rods and/or tubes are stacked in a close-packed arrangement to form a preform, that can be drawn into fiber using a conventional tower setup.
Most optical fiber manufactured today includes a first coating (so-called “primary coating”) surrounding and in contact with the cladding glass portion of the fiber, and a second coating (so-called “secondary coating”). The primary coating in older fibers typically had a Young's modulus of about 3 to 5 MPa, while the primary coating modulus of more recent fibers is typically 0.6 to 2 MPa, and the secondary coating in older fibers typically had a Young's modulus of about 400 to 600 MPa, while the secondary coating modulus of more recent fibers is typically about 1000 to 2000 MPa. Primary coatings in known fibers have been utilized specifically to reduce signals losses because of stresses in the fiber induced by microbending of the fiber.